This invention relates to immersion heating elements, for use in liquid heating vessels such as electric kettles, deep fat fryers, coffee makers and the like. In particular, the invention concerns heating elements which provide heating using thick film heating tracks.
The use of thick film heating elements is increasing in hot water applications such as kettles and coffee makers. Conventionally, a thick film heating element defines the base of the vessel for the liquid to be heated. The heating element itself comprises a metal substrate on one side of which there is provided an insulating layer, and a printed circuit heating track is provided over the insulating layer. When the heating element is mounted in the vessel, the side of the substrate opposite the heating track defines the base of the vessel, so that the heating track and insulating layer are disposed outside the vessel and thereby do not make contact with the liquid to be heated. The use of thick film heating tracks enables the power of the heating element to be increased, and the additional advantage is provided that cleaning the inside of the vessel is easier, when compared with vessels using conventional sheathed immersion heating elements. A flat heating element also provides improved appearance of the inside of the liquid heating vessel.
There have also been proposals to manufacture a thick film immersion heating element. For example, International Patent Publication No. W094/18807 discloses an immersion heating element comprising a thick film resistive heating track which is deposited between insulating layers and provided on a stainless steel support plate. The heating track must be covered by an insulating layer because, for an immersion heater, both sides of the heating element are in contact with the liquid to be heated. The top insulating layer must also function as a mechanical protection layer, and therefore requires sufficient thickness to resist incidental damage (for example during cleaning).
For a conventional thick film heating element, which defines a portion of the vessel for the liquid to be heated, the insulating layer is fired at a first temperature on the metal substrate, and the thick film heating track is deposited over the insulating layer and fired at a second, lower temperature. Consequently, the firing stage for the heating track does not damage the insulating layer beneath, since it can withstand much higher temperatures, and has a higher firing temperature itself. The problem therefore arises if an attempt is made to provide an additional insulating layer over the heating track, as suggested in WO 94/18807, that the firing required to bond the second insulating layer over the heating track may damage the heating track itself, for example changing the resistance of the heating track, if similar insulating layers are selected.